1. Technical Field
The present disclosure relates generally to radio frequency (RF) circuits, and more particularly, to low dropout voltage regulators for highly linear radio frequency power amplifiers.
2. Related Art
Complex, multi-function electronic devices are comprised of many interconnected modules and components, each of which serves a dedicated purpose. As a general example, wireless communication devices may be comprised of a transmit chain and a receive chain, with the antenna and the transceiver circuit being a part of both the transmit chain and receive chain. The transmit chain may additionally include a power amplifier for increasing the output power of the generated radio frequency signal from the transceiver, while the receive chain may include a low noise amplifier for boosting the weak received signal so that information can be accurately and reliably extracted therefrom.
The low noise amplifier and the power amplifier may together comprise a front end module or front end circuit, which also includes a radio frequency switch circuit that selectively interconnects the power amplifier and the low noise amplifier to the antenna. The connection to the antenna is switched between the receive chain circuitry, i.e., the low noise amplifier and the receiver, and the transmit chain circuitry, i.e., the power amplifier and the transmitter. In time domain duplex communications systems where a single antenna is used for both transmission and reception, this switching between the receive chain and the transmit chain occurs rapidly many times throughout a typical communications session. Besides radio frequency communications systems, switches and switch circuits find application in many other contexts.
The radio frequency switches and the amplifier circuits of the front end module are manufactured as an integrated circuit. Although the gallium arsenide (GaAs) or silicon-on-insulator (SOI) fabrication technologies were once favored for devices for high-power applications such as cellular communications systems and wireless local area network client interface devices, complementary metal oxide semiconductor (CMOS) fabrication is becoming increasingly mainstream for its lower manufacturing costs.
Radio frequency amplifier circuits ideally have a linear performance in order to meet the operational requirements of the wireless communications standards with which they must conform. CMOS transistors, however, are prone to a low breakdown voltage. Low dropout voltage (LDO) regulators may be embedded in the integrated transceiver circuitry as well as the radio frequency front end circuits, as reliable operation is possible with a limit voltage that exceeds the transistor voltage, while the external bias supply voltage (typically a battery) varies over a wide range. The internal voltage at the low dropout voltage regulator output follows the battery voltage, and at low battery voltage levels, the output is adjusted to have a small drop-out voltage. The low dropout voltage regulator also has a maximum internal voltage for reliable CMOS transistor operation, and is kept at a fixed level if the battery voltage is increased beyond a maximum. The drop out voltage is increased in this case, and is understood to be the equivalent of inserting a resistor at the output of the low dropout voltage regulator in series with a load.
A low dropout voltage regulator typically requires an input capacitor and an output capacitor to ensure proper operation, and perform within acceptable stability, load response, and input response parameters. For the transceiver circuitry, due to lower current consumption, such capacitors may have small values, typically in the ten pico-Farad to a couple hundred pico-Farad. Such small value capacitors may be readily integrated on the semiconductor die, though because of their substantial footprint on the die and additional consequent costs, external capacitors may be used instead.
For high current consumption circuits inside the transceiver or the radio frequency front end such as the power amplifier, a capacitor having a higher value in the range of several nano-Farads may be required. Implementing such a high capacitance is not possible on-die. Furthermore, when utilized in connection with radio frequency power amplifiers, as the drop out voltage of the regulator increases, linearity is degraded. In order to counteract the increased equivalent series resistance with the load, a higher value output capacitor may be necessary to maintain acceptable linearity.
Since on-die implementation is not possible, an external capacitor is necessary. Yet, in the highly miniaturized transceiver and front end circuits that are required in modern wireless communication devices with the smallest footprints, the addition of an external capacitor may be impractical, not only in terms of circuit board real estate, but also because of the limited pin count. Pin-to-pin compatibility issues may restrict integrated circuit pinout configurations for additional external capacitors, and is likely to impact circuit board layouts.
Accordingly, there is a need in the art for an improved low dropout voltage regulator that can be integrated on the same semiconductor die as the radio frequency amplifier circuitry, while maintaining a high level of linearity for the same. There is also a need for such circuits to avoid inclusion of an external low dropout voltage regulator output pin.